Optical: systems and elements – Diffraction – From grating
Patent
1996-04-12
1999-06-15
Henry, Jon
Optical: systems and elements
Diffraction
From grating
359 2, 359572, 359574, 283 91, 283 93, 283902, G02B 518, B42D 1510
Patent
active
059127678
DESCRIPTION:
BRIEF SUMMARY
This invention relates to diffractive indicia for a surface. It relates particularly to optically variable inks and optically variable elements which appear to change colour when the viewing angle and/or the angle of illumination is varied.
An important feature of several documents such as bank notes, credit cards, cheques and share certificates, is that they be difficult to reproduce by way of forgery. Because the range of available reproduction equipment and techniques is constantly improving, there is a continuing need for more and better anti-forgery security techniques.
One such anti-forgery security technique involves using indicia which incorporate optical variability. An optically variable document has an appearance which changes when the viewing angle and/or the angle of illumination changes. It is easy for a forger to reproduce a document which contains optically invariable images, but it is extremely difficult for a forger to incorporate optical variability into a reproduced document. One well known example of an optically variable device is the hologram diffractive device commonly used on credit cards.
A less well known type of security device is a colour switching ink provided by Sicpa Holding SA. After the ink has been applied to the surface of a document, it appears to be green when viewed from particular angles relative to a light source, and red when viewed from other angles, so that particular movements in viewing angle, lighting direction or document orientation result in a colour switch effect from green to red or red to green. It is believed that the colour switching Sicpa ink comprises a large number of small multi-layer interference elements. The layers have different refractive indices, and partial reflection occurs at the interface between each layer. The layer thicknesses, refractive indices and degrees of reflectiveness are chosen such that an optical interference effect occurs between the layers such that, for a given illumination direction, red light is reflected in a particular range of directions and green light is reflected in another range of directions. The colour switching ink provides an effective security device, but it is very difficult to make the multi-layer interference elements, and consequently, the ink is very expensive.
According to the present invention, there are provided diffractive indicia for a surface, comprising: diffractive elements, when applied to the surface, are not separately resolvable to the human eye; and changes when the viewing angle and/or angle of illumination relative to the surface changes.
In one preferred aspect of the invention, the diffractive indicia are incorporated in an ink, with the diffractive elements dispersed throughout the ink, the means for adhering is an ink binder which fixes the diffractive elements on the surface as the ink dries.
The ink binder may be any suitable binding agent. An ink is typically in the form of a liquid or a paste having a relatively volatile solvent which evaporates after the ink has been applied to the surface of a document. Ink also typically includes one or more colouring substances and a binding agent, which fixes the colouring substances in position on the document as the solvent evaporates.
The diffractive elements may be of any suitable shape and configuration. Each element has a surface which is embossed with a diffracting surface structure, and it is preferred although not essential that each element be in the form of an embossed foil, arranged such that the embossed surface structure is the same on-both sides. It is further preferred that the diffraction elements be substantially disc-shaped, although they may be of any suitable shape including square, triangular, rectangular, hexagonal and irregularly shaped.
The diffractive elements may be of any suitable size. It is preferred that the diffractive elements have linear dimensions of 30 micron or less, most preferably with linear dimensions of the order of 10 micron. A smaller size allows greater dispersion throughout the ink, but also reduc
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Commonwealth Scientific and Industrial Research Organisation
Henry Jon
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